Most automotive vehicles are propelled by internal combustion engines consuming hydrocarbon fuels. Burning these fuels produces exhaust gas containing harmful air-pollutants, such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons. It also contains substantial amount of carbon dioxide which, if produced in large quantities worldwide over long period of time, can contribute to an undesirable increase in average global temperature. Concern for clean air and a desire to prevent adverse consequences of man-made global warming dictate a need to substantially improve fuel efficiency of automotive vehicles.
By itself, the internal combustion engine is a reasonably efficient machine. Unfortunately, the driving pattern of most automotive vehicles is such, that a substantial fraction of energy produced by their engines is wasted. Typically, the driving pattern involves frequent accelerations, each followed by a deceleration. Each acceleration involves a significant increase in fuel consumption needed to produce the additional energy necessary to increase the vehicle speed. Then, during a subsequent deceleration, this added energy is absorbed by vehicle brakes and dissipated as heat.
Attempts to overcome such waste of energy led to development of systems in which the energy of vehicle motion is not dissipated during braking, but converted into a form in which it can be temporarily stored and, then, used again to accelerate the vehicle at a later time. Typically, such system includes an internal combustion engine, an energy storage, and a second machine absorbing the energy of vehicle motion and placing it into the storage during braking. During subsequent acceleration, the second machine receives energy from the energy storage and uses it to supplement the work of the internal combustion engine. Such systems are known as hybrid vehicle systems. An electric hybrid includes an electric generator/motor as a second machine, and an electric battery for energy storage. A fluid-power hybrid includes a pump/motor and a pressurized-fluid accumulator. A flywheel hybrid includes a variable-ratio transmission and a flywheel.
A disadvantage, common to all of the above mentioned hybrids, is added cost and complexity associated with the need for the second machine and associated mechanisms needed to connect it to vehicle wheels in-parallel to or in-line with the internal combustion engine. Added complexity also increases probability of failures, thus contributing to a reduction in overall system reliability.
Another significant disadvantage is a substantial increase in vehicle weight, which is especially pronounced in hybrids using electric batteries for energy storage. Electric batteries are excellent energy storage devices, but the weight of their electrode-plates and electrolyte often adds so much to the mass of the vehicle that it requires a larger engine to drive it. In addition, a heavier vehicle is likely to cause more damage in traffic accidents.
Another deficiency of hybrids using a second machine is that the process they use for energy conversion is often inefficient. For example, one-way energy conversion efficiency of many conventional electric generators and motors does not exceed 50%, and therefore, at best, only a quarter of braking energy can be reclaimed for acceleration. More advanced generators and motors have higher efficiency, but their cost is often prohibitive.
A very significant drawback of electric batteries is a relatively slow rate at which they can be efficiently charged. This limits their ability to absorb the vehicle braking energy during a strong deceleration. In addition, the durability of electric batteries is much less than the life of an average vehicle. Changing the battery, during the life of the vehicle, adds to the maintenance costs.
In view of the above, it is clear that it is highly desirable to have a vehicle system which does not suffer from the above disadvantages, while retaining all the fuel economy advantages of other hybrid systems. A properly conceived hybrid system using compressed-air for energy storage can meet these requirements. Such a system is the subject of the present invention.